Your booking page shows the remaining balance with a payment button.
Deposits are usually taken when a campsite makes a booking for you, for example over the phone. Your booking page shows what you've paid and what's left.
When there's a balance to pay, the booking page has a payment button that takes you to the same secure Stripe page you'd use for any booking. The campsite may also tell you a date the balance is due.
The link to your booking page is in your confirmation email. See Where do I find my booking?.
Email support@pillowcase.io with your campsite name or booking reference and we'll pick it up from there.